Breast milk contains maternal antibodies (MatAbs) that promote infant health and moderate diarrhea, and soluble immunomediators (TGF2, IL4, sCD14, etc) whose role is largely unexplored. Rotavirus (RV) is a leading cause of diarrhea in infants worldwide. Newly licensed oral RV vaccines have unproven or lower efficacies in developing countries and high costs remain obstacles to universal adoption. Although lactic acid bacteria (LAB) reduce RV diarrhea in infants, mechanisms are undefined and the effect of providing concurrent MatAb/mediators with LAB is unexplored. Our innovative studies will first assess the impact of colostrum (col)/milk and key maternal immunomediators (TGF2, IL4) on colonization by probiotics. We then determine the individual and combined effects of col/milk and LAB on immunologic maturation, moderation of RV diarrhea and homeostasis in the neonatal gut. Bifidobacteria and lactobacillus strains differ in breastfed vs formula fed infants. We hypothesize that col/milk promotes colonization by these 2 probiotic LAB, requiring fewer or lower LAB doses, and this synergism enhances gut immune maturation and homeostasis leading to moderation of rotavirus diarrhea. Preliminary studies showing that L rhamnosus GG (LGG) colonized col/milk-fed pigs long-term after a single oral dose support our novel idea. Our unique neonatal gnotobiotic (Gn) pig model is colostrum- deprived (no MatAb pre-col/milk), allowing MatAb/mediator manipulation and free of microbes (caesarean-derived, sterile housing), permitting evaluation of defined probiotics. Importantly their gastrointestinal physiology, mucosal immune responses and susceptibility to human RV diarrhea mimic that of infants. In Aim 1, we determine if col/milk containing MatAb/mediators (or TGF2+IL4) influence a) Bifidobacterium lactis and LGG colonization, persistence and distribution in the gut and b) maturation of neonatal gut immune responses. In Aim 2 we elucidate a) if col/milk with low titer RV antibody (naturally exposed sows) in concert with LAB modulates HRV pathogenesis and immunity, thereby moderating HRV diarrhea and b) if an enteropathogenic viral infection affects gut homeostasis and the LAB microflora. We will use in vivo bioluminescent imaging to study LGG interactions with/without B. lactis in the neonatal gut, and the influence of col/milk or HRV infection on these interactions. This novel approach to in vivo bacterial ecology will elucidate probiotic LAB interactions within the host. Intestinal and systemic immunologic parameters to be assessed include: 1) innate responses;2) virus- specific, LAB-specific and total isotype B cell responses focusing on gut IgA;3) T cell responses (Th1, Th2, Th3, Tr1 cytokines). Our innovative studies will provide improved understanding of maturation of neonatal immunity and new insights into the beneficial effects of novel col/milk MatAb/mediators in concert with probiotics in protective immunity to RV. Further, our findings will suggest new strategies to limit RV diarrhea in infants in developing countries through short-term co- administration of probiotics and col/mlk mediators to breastfed infants or pregnant mothers. This approach will promote stable LAB colonization and gut immune maturation in infants leading to disease moderation. Immunomodulation and enhancement of neonatal immune responses also have fundamental implications for tolerance to food antigens or commensals to control food allergies or inflammatory bowel syndromes as well as antibiotic-induced diarrheas.